1. Field of the Invention
The present invention relates to a pneumatic tire, and more specifically a pneumatic tire that is excellent in both wet traction and wear resistance.
2. Description of the Related Art
In order to improve tires in terms of a driving stability on wet roads and a braking ability on snowy and icy roads (hereinafter, these abilities are collectively referred to as “wet traction”), there has been heretofore a widely used practice in which sipes extending in a tire width direction are formed in tread land portions to produce an edge effect of improving drainage and snow clearing of the tires. In this regard, if an increased number of sipes or deeper sipes are formed in tread land portions, the tires have a problem of decrease in the stiffness of the land portions, and accordingly decrease in the wear resistance on dry roads. On the other hand, if the depth of sipes formed in the tread land portions is decreased, the tires are advantageous from the viewpoint of the wear resistance, but have another problem that, when the wear of the tires reaches an advanced stage, the wet traction is sharply decreased due to disappearance of the sipes.
Thus, the wet traction and the wear resistance are in a trade-off relationship as described above, and various proposals have been heretofore made to overcome the trade-off problem. One of the approaches to this issue is disclosed in a proposal to arrange block rows in a tread portion and to form a sipe in the surface of each of blocks included in each block row so that the sipe can traverse the block and open to main grooves. In this proposal, specifically, the sipe is formed to be shallow at its end parts on the opening sides and to be deep at its center part for the purpose of ensuring the stiffness of the block, and is also formed to be wide at the end parts on the opening sides for the purpose of ensuring the drainage (for example, Japanese patent application Kokai publication No. 2001-233021). Another proposal in this approach is that a block is formed to be narrow at its front and rear parts in a tire circumferential direction in a planar shape and to have a recessed portion in its center position in the tire circumferential direction (for example, Japanese patent application Kokai publication No. 2007-153275).
In these proposals, however, a sipe is formed to have shallow end parts on the opening sides. For this reason, when the wear reaches an advanced stage, the sipe on the opening sides disappears and thereby rain water or the like trapped in a center part of the sipe has nowhere to escape. As a result, these proposals have a problem of sharp decrease of the wet traction, and, in fact, do not provide sufficiently effective measures for achieving excellent wet traction and wear resistance together.